Being in possession of level 6 does not mean you have an accent or pronunciation authentic. I can assure you that many British citizens have a pronunciation or accent that does not deserve a level 6. But of course ... as they will give you a level below 6 being British, right?
Many people fron UK or USA do not vocalize well and understand them is a problem. Looks like they're chewing gum when speaking or hiccuping.
Paradoxically, they should set a good example.